I've installed an app from the Windows Phones Store called "360手机卫士", a so-called famous(or infamous) antivirus software in china, which is said to be able to check your installed apps. After using it, I found that during the scan process,it does get the list of all my apps and scanned them. So after checking on the internet I get to know usually apps can't get the list because of "sandbox". How did it passed through or how did this app made it?
BTW：the download of this app. Make sure you use WP8.1GDR2 if you are afraid of it peeking your contacts. http://www.windowsphone.com/s?appid=245303df-88cc-44d9-83ae-dc687fea0b28
Getting info about other apps requires some special/restricted capabilities that are only available to apps that are developed by Microsoft and its partners (Qihoo 360 is one of MS's partners). I can't find reference for WP but this is what I find about Windows Store app.
App capability declarations- there is an Other App Mangement capability in the restricted list.
The packageManagement restricted capability allows apps to manage other apps directly. The packageQuery device capability allows apps to gather information about other apps.
And
... Restricted capabilities are available only to apps that are developed by Microsoft and its partners.